Important Concepts
1. Secondary haloalkanes undergo slow and tertiary haloalkanes undergo fast unimolecular substitution in polar media. When the solvent serves as the nucleophile, the process is called solvolysis.
2. The slowest, or rate-determining, step in unimolecular substitution is dissociation of the C – X bond to form a carbocation intermediate. Added strong nucleophile changes the product but not the reaction rate.
3. Carbocations are stabilized by hyperconjugation: Tertiary are the most stable, followed by secondary. Primary and methyl cations are too unstable to form in solution.
4. Racemization often results when unimolecular substitution takes place at a chiral carbon.
5. Unimolecular elimination to form an alkene accompanies substitution in secondary and tertiary systems.
6. High concentrations of strong base may bring about bimolecular elimination. Expulsion of the leaving group accompanies removal of a hydrogen from the neighboring carbon by the base. The stereo chemistry indicates an anti conformational arrangement of the hydrogen and the leaving group.
7. Substitution is favored by unhindered substrates and small, less basic nucleophiles.
8. Elimination is favored by hindered substrates and bulky, more basic nucleophiles
From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE
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